You’re right. The “Fill”-button is only there for “auto-fill suggestions” and though that can be login items, cards and identities, the “Fill”-button is by default not there for ALL cards and ALL identities.
Therefore, activating the option to change to the “old” autofill behaviour, only works for the auto-fill suggestions where the “Fill”-button would be…
Here is a new feature request that might fit in here: Restore "Show Cards" and "Show Identities" options in browser extension (look also closely at the second post in there, written by @grb - there seem to be also detection issues for the auto-fill suggestion of cards and identities…)